1. Field of the Invention
The invention is directed to a method for determining the insulation resistance in a terminated, fully insulated power supply network, a so-called IT network. IT networks of this type are used, for example, in vehicles.
2. Description of the Related Art
An IT network in accordance with the prior art has at least one power source, preferably a generator, at least one rectifier, an intermediate circuit having at least one capacitor, at least one converter and a measuring arrangement for the intermediate circuit voltage. An IT network is generally understood to be a network configuration in which the feeding power source is not grounded as usual. Consequently, in the event of a fault, such as an insulation fault, no closed electric circuit is established nor can any dangerous current flow through a body take place. Equally, in the event of a first fault, the operation of the IT network does not have to be stopped, which brings about a higher protection against failure. What is disadvantageous here, however, is that such a fault is also not identified as long as the insulation to ground is not measured by an insulation monitoring device.
On account of the above-described properties of IT networks, monitoring of the insulation at the beginning of operation and also during operation is particularly preferred. In this respect, EP 0 751 396 B1 describes a method for monitoring the insulation for faults, but implementation of the method requires a considerable additional expenditure on circuitry over and above that already present in the IT network.
Changes in the insulation resistance in an IT network also cause faults in the network, and have to be reliably identified by monitoring the network. Such faults include, by way of example, the ageing of the insulation, which results in a slow change in the insulation resistance that is often even symmetrical over all the phases. Another known fault case stems by way of example from damage to insulation, which brings about a faster change in the insulation resistance, usually of only one phase.